The measurement of root-mean-squared (RMS) values in both AC and DC waveforms has been substantially simplified in recent years through the development of RMS microchips, such as the BURR BROWN 4340 and 4341 True RMS-to-DC conversion integrated circuits (BURR BROWN is a registered trademark of the Burr-Brown Research Corporation principally located in Tucson, Ariz.). These units, which are each a single microchip, provide a DC output equal to the RMS value of the input waveform. When the time over which the measurement is being made is long compared to the individual perturbations of the waveform, the averaging time is substantial and the output is a very precise measurement. However, as the averaging time is shortened, and the duration of each perturbation approaches the overall measurement time, the measured value becomes less accurate.
Previously, the RMS measurement of waveforms having a short time duration and only a few perturbations was made using special dedicated RMS circuits. U.S. Pat. Nos. 3,201,688 and 3,289,079, which are incorporated herein by reference, disclose systems for measuring waveforms of a short duration where the base frequency of the perturbations is known and fixed. Using such circuits, RMS measurements can be made on a single perturbation or pulse, while relating the RMS value to waveforms having a plurality of similar perturbations with the same fixed base frequency. However, when the base frequency of the perturbations is not known or is variable, the circuits disclosed in these patents are not suitable. Additionally, such circuits operate properly only when the waveform to be measured is sinusoidal, or nearly sinusoidal.
The measurement of RMS values of current in high current heating and welding circuits requires special consideration. The signal may typically be measured by using a high current shunt, a Hall effect transducer, a current transformer, or an air core toroid coil (or Rogowski coil or belt). All of these methods have limitations. The high current shunt is expensive, difficult to mount in most systems, and because it is physically large, the shunt disturbs the circuit by increasing the physical size of the system. The Hall effect transducer is position sensitive, temperature sensitive and has a limited linear range. The high current transformer is expensive, bulky, has a limited linear range, and disturbs the circuit. The air core toroid coil, however, is economical, small in size, easy to locate in the circuit, disturbs the circuit very little, is linear over a very large range, but, however, provides an output proportional to the first differential with respect to time of the current in the monitored circuit. Therefore, in using air core toroid coils, it is usually necessary to integrate the first differential waveform generated by the air core toroid coil in order to obtain the current waveform before the RMS value is determined.
There are many instruments which measure RMS values of short duration AC pulses of high current at power line frequency using an air core toroid coil to obtain an input signal. In all cases, some form of electrical integration is used prior to deriving the RMS value. The only exception occurs where the waveform is a pure sine wave yielding a cosine wave output from the air core toroid coil. In this case, the differential is equal to the initial waveform except for a shift in phase.
There are generally two types of integrators, passive and active, each of which have advantages and limitations. Passive integrators are very stable but have very low output. Active integrators have high output, some DC drift, limited frequency range, and, when used for integration of signals with very low frequencies, often have an output that has low frequency oscillation resulting in a "rocking" output. In order to obtain RMS measurements, active integrators are primarily used followed by some form of dedicated RMS integrated circuit.
In most presently used high current systems, the current level is controlled by using a high current silicon controlled rectifier (SCR) switch to turn the current system on a short time after each half cycle of power line voltage has begun. This system, however, yields a current output for less than a full half cycle for each half power line voltage cycle. Since the measured RMS current value is used in controlling the current to the circuit, a device that generates RMS current values must be accurate. In order to obtain accurate RMS current values, the current must be measured for the whole half cycle, not merely that portion of the cycle when the current is not equal to zero. If the RMS current is determined during a half cycle only when the current is non-zero, the resultant RMS current value is greater than the true RMS current value, and accurate control of the current through the circuit cannot be achieved.